The Nafion membrane developed by DuPont in 1966 as a proton conductive membrane has been almost the only advanced polymer electrolyte available for use in a membrane electrode assembly in a fuel cell.
Due to the extreme requirements concerning the chemical stability of polymer electrolyte membranes used in fuel cells, only a very small number of polymer backbones are promising candidates. Aromatic polyethers are among the first choice for this application. Only a very limited number of polymer backbone types have a chance of withstanding the extremely demanding conditions a fuel cell membrane is exposed to. Hydrolysis, oxidation and reduction (hydrogenation) can lead to degradation of the polymer, reducing the lifetime of a polymer electrolyte membrane. Poly(arylene ether)s, especially poly(ether ketone)s and poly(ether sulfone)s, as well as polyarylenes such as poly(para-phenylene) are the most promising candidates for future improvements. Besides their chemical stability, these polymer classes also exhibit excellent mechanical properties in their native forms.
U.S. Pat. No. 4,625,000 describes a sulfonation procedure of poly(ether sulfone)s for solid polymer electrolytes. However, post sulfonation of preformed polymers offers little control of the position, number, and distribution of the sulfonic acid groups along the polymer backbone. Moreover, the water uptake of membranes prepared from post sulfonated polymers increases leading to large dimensional changes as well as a reduction in strength as the degree of sulfonation increases.
U.S. Pat. No. 6,090,895 provides a process for making cross linked acidic polymers of sulfonated poly(ether ketone)s, sulfonated poly(ether sulfone)s, sulfonated polystyrenes, and other acidic polymers by cross linking with a species which generates an acidic functionality. However, this reference does not suggest an effective way to cast membranes from those cross linked sulfo-pendent aromatic polyethers.
EP Patent No. 1,113, 517 A2 discloses a polymer electrolyte containing a block copolymer comprising blocks having sulfonic acid groups and blocks having no sulfonic acid groups by post sulfonation of precursor block copolymers consisting of aliphatic and aromatic blocks. In this patent, the precursor block copolymers are sulfonated using concentrated sulfuric acid, which leads to the sulfonation of aromatic blocks. However, this post sulfonation of aromatic blocks offers the little control of the position, number, and distribution of the sulfonic acid groups along the polymer backbone and this post sulfonation of precursor block copolymers also leads to the cleavage of chemical bonds of an aliphatic block.